What happened
On the afternoon of September 3 1978, a pilot launched a glider flight from Montricher in the Astir CS, registration HB-1309. The flight was intended to be a high-altitude soaring mission, and the pilot successfully reached an altitude of 5,400 meters. During the flight, the aircraft encountered extreme temperatures of -22°C at high altitude.
As the pilot began a descent through the Ballaigues region, the aircraft encountered deteriorating weather conditions, including snow and low visibility. While flying at approximately 1,500 meters, the intense cold caused significant internal icing on the cockpit canopy and the mirror used to read the compass. This icing prevented the pilot from clearly seeing the compass rose. Consequently, the pilot intended to fly a heading of 090 degrees toward Orbe but instead inadvertently turned to a heading of 270 degrees. This loss of orientation led the aircraft directly into the snowy terrain of the Jura mountains near Bel Coster. The glider struck the ground at 18:07 local time, resulting in the destruction of the aircraft, though the pilot escaped without injury.
The investigation
SUST investigators examined the aircraft, the pilot's credentials, and the meteorological conditions at the time of the accident. The investigation confirmed that the pilot held a valid license and possessed significant flight experience, including extensive hours in both motor and instrument flight. The Astir CS was found to be in a well-maintained condition with valid certificates, and the aircraft's weight and balance were within limits.
The investigation focused on the environmental factors, noting that while the ground temperature was approximately 8°C, the temperature at the accident site's higher altitude was near 0°C. This explains the presence of snow on the ridge and the icing on the aircraft's cowling. The investigators also analyzed the cockpit's optical setup, noting that the pilot's semi-reclined position made it difficult to read the compass directly, relying instead on a mirror that had been rendered unreadable by frost and condensation.
Findings
- The primary cause of the accident was the pilot's loss of orientation due to the inability to read navigation instruments.
- Severe internal icing on the cockpit canopy and the compass mirror significantly degraded external visibility and instrument legibility.
- The flight was conducted in marginal meteorological conditions, characterized by falling snow, low cloud bases, and poor visibility.
- The pilot's error in heading (a 180-degree deviation from the intended course) was a direct result of the obscured compass mirror.